1. Field of the Invention
This invention relates to and has among its objects the provision of a novel method and apparatus for dewatering an alcohol-water solution. Parts and percentages are by weight unless otherwise specified.
2. Description of the Prior Art
The production of gasohol by the blending of fuel grade ethanol with gasoline has the potential for helping meet energy needs. Alcohol blends with gasoline require 99.35 percent alcohol to minimize water separation problems during distribution, storage and use. To make effective use of ethanol as a substitute fuel the energy consumed to make the fuel grade alcohol must be less than the energy obtained from ethanol (84,090 Btu/gal or 7120 cal/g).
The conventional method to concentrate an aqueous solution of ethanol involves two steps: first, a dilute ethanol-water mixture (6-12 percent ethanol) is distilled to about 95 percent; next, the solution of step one is azeotropically distilled to anhydrous alcohol having a concentration of about 99.8 percent. Distillation to concentrations above 85 percent are energy intensive because the vapor and liquid phases are very similar in composition. Full separation of vapor and liquid phases having similar composition requires both a large reflux with attendant large energy load in the rectification section of the distillation column and a large number of equilibrium contacting stages resulting in a device which is physically tall and costly. Furthermore, simple (non-azeotropic) distillation is limited with regard to ethanol-enrichment because the alcohol-water mixture forms a constant boiling azeotrope at 95.6 percent ethanol.
The theoretical amount of energy expended to distill ethanol from 5 to 100 percent calculated by balancing heat input into the system and heat lost is about 3420 cal/g. In industrial practice, the actual energy expended during distillation is lower than theoretical due to the inclusion of various heat recovery systems. The reported loss of the fuel value to distill from 10 percent to 95 percent ethanol in industrial practice is about 13-21 percent; the loss of fuel value to concentrate from 95 to 100 percent by azeotropic distillation with benzene is an additional 7-11 percent. Overall expenditure is about 1400-2400 cal/g. The capital cost to produce 100 percent ethanol with an expenditure of only about 1400 cal/g is nearly double that of a distillation plant producing 95 percent ethanol due to the inclusion of azeotropic distillation equipment and advanced design heat recovery systems.
Several alternate approaches to obtain anhydrous ethanol which eliminate the energy costly azeotropic distillation have been suggested. These include dehydrating ethanol with such materials as gypsum, calcium chloride and lime, molecular sieves, biomass materials or the like, or solvent extraction. One technique involves the use of sorbents to selectively adsorb water from an ethanol-water mix. In the Purdue process (Chemical Engineering, Vol. 87, p. 103, Nov. 17, 1980), ethanol-rich vapors (80-92 percent ethanol) leaving a first stage distillation at a temperature of about 78.degree.-80.degree. C. are passed directly onto a column of cornmeal to adsorb water and obtain anhydrous ethanol. After the column is saturated, the cornmeal is regenerated by passing hot (90.degree.-120.degree. C.) air over it; simultaneously, a second previously regenerated column is brought into operation. Overall energy expenditure for the distillation and sorption processes including the distillation step is about 1000 cal/g.
Disadvantages of this method are that regeneration of the cornmeal sorbent is required, ethanol adsorbed onto the cornmeal is not recovered, the sorption process must be physically connected to the distillation operation, and fusel oils (amyl acetate) generated during the distillation process pass onto the column and may lead to reduced efficiency. Furthermore, since the capacity of cornmeal to adsorb water decreases as the temperature is increased from ambient, efficiency of the sorbent is less at the temperature of the process than if the operation was carried out at lower temperatures.